The standard TIA/EIA IS-95 defines channel coding carried out for the signal in a base station of a cellular radio system. The standard defines the channel coding employed in data transfer on an Air interface between a base station and a subscriber terminal of a cellular radio system. The standard defines the channel coding to be carried out in a channel coder of the base station. In the channel coder defined by the standard, the signal is CRC block-encoded (CRC=Cyclic Redundancy Check), convolutional-coded and interleaved. The interleaving is followed by converting the signal into an RF signal and transmitting it on the radio path to the subscriber terminal.
The standard TIA/EIA IS-96 determines a variable-rate speech encoder employed in the cellular radio network. As the variable-rate encoder, for example a CELP vocoder (CELP=Code Excited Linear Predictive) has been used. The standard defines PCB (PCB=Parity Check Bits) and CRC calculation for several different data rates, such as 1200 kbit/s, 2400 kbit/s, 4800 kbit/s and 9600 kbit/s. In addition, the standard defines signal interleaving for all the abovementioned data rates. The PCB calculation has in the standard been defined to be carried out in the vocoder in connection with the source coding. In prior art, the vocoder is in the TRAU unit (TRAU=Transcoder/Rate Adaptor Unit), located apart from the base station. The TRAU unit is located, for example, at the base station controller or the mobile services switching center.
The TRAU unit is used in signal source coding, adapting the data rate to be suitable for the transfer network, e.g. the PSTN network (PSTN=Public Switched Telephone Network), and for transmitting TRAU frames to the base station. The TRAU unit codes the 64 kbit/s signal originating from the PSTN network to a 16 kbit signal, whereby the user data rates are at the aforementioned levels. The TRAU unit serves as the source coder, reducing the bandwidth on the Air interface between the base station and the subscriber terminal.
The prior art TRAU frame is, e.g. 192 bits long before and after the convolutional coding. The convolutional coding is carried out in the channel coder. It takes 20 ms to transmit a frame. The frame comprises, e.g., 172 information bits, 12 CRC bits and 8 tail bits. In the aforementioned case, the data rate is 9600 kbit/s. The frame comprises one MM bit (MM=Mixed Mode Bit). The remaining part of said frame, i.e. 171 bits, is allocated for traffic.
In prior art, the calculation of PCB bits, the coding and decoding of PCB bits is carried out in the vocoder i.e. the TRAU unit. The calculation, coding and decoding of the bits requires bit buffering, which causes delay. The CRC block encoding is performed at the base station in connection with channel coding. In calculating CRC block encoding and bit interleaving, additional buffering is required, resulting in additional delay. In addition, the prior art solution takes up a lot of transmission path capacity.